In the drilling of deep wells in earth formations many different types of earth formations are encountered. Boring or drilling operations in each of these types of formations typically requires drill bits of differing character. For example, in loose or unconsolidated earth formations such as gravel strata, soft sandstone, it is desirable to utilize drill bits having a plurality of fixed blades for cutting away the formation to define a borehole. Where consolidated, very hard earth strata is encountered, for example in deep earth formations, it is typically desirable to employ drill bits having a plurality of rotary cutters each having a multiplicity of cutter teeth thereon. Many drill bits of this nature are referred to as "tri-cone rock bits". Typically this type of rock bit incorporates a drill bit body having three and in some cases more depending legs or lugs each defining a spindle. Typically each of the spindles projects toward the rotary axis of the drill bit body. Each of the spindles then supports a rotary cutter element or "cone" which is provided with a multiplicity of cutter teeth. The cutter teeth of individual cutter cones are cooperatively associated or arranged with the cutter teeth of other cutter cones of the drill bit so that the earth formation is cut away as the drill bit is rotated at the lower extremity of drill pipe that is rotated by a drilling rig.
One of the paramount disadvantages of rotary cone type drill bits having cone type rotary cutters is the inability of the cutter bearings or bushings to withstand the severe wear characteristics to which the drill bit is typically subjected. As drilling operations occur, rock bits are subjected to severe impacting and vibration as well as other wear inducing factors that are highly detrimental to the service life of the bearings or bushings of the rotary cutter cones. When tri-cone rock bits are rotated for extended periods of time under high loading, even though provided with bearings or bushings for cutter cone support the bearing surfaces of the spindles can quickly become worn to an essentially elliptical configuration, permitting the cutter cones to wobble or tilt due to the excessive clearance that occurs as the result of wear. Further, rotation of the cutter cones in the presence of drilling fluid which is of extremely abrasive nature will cause rapid bearing seal deterioration permitting invasion of the abrasive drilling fluid into the bushing or bearings of the cutter cones. When this occurs, the bearings or the spindle of the drill bit will become rapidly worn to the point that the cutter cones can become separated from the spindles, whereupon they can fall into the drilled bore when the drill bit is lifted and thus interfere with drilling operations and will typically require that the drill bit be withdrawn from the hole and the cutter cones retrieved by fishing operations. This of course is expensive and time consuming and results in significant cost that becomes a detriment to the overall cost of the drilling operation. It is desirable therefore to provide a rotary cone type rock bit having a bearing or bushing assembly that provides for rock bent cutter retention and also serves to enhance the wear resistant characteristics of the drill bit assembly.
Typically the drill bodies of rotary cutter bits take the form of integral cast or forged structures that are very expensive to manufacture because of the complex configuration thereof. It is also desirable to provide a rotary drill bit construction having a body structure of exceptional strength and durability and yet being of relatively low cost because of the provision of efficient bushings or bearing assemblies that efficiently accommodate the severe wear characteristics to which the rotary cones are subjected and also insure against wear of either the spindles or the rotary cones of the drill bit construction so that loss of the cutter cones is significantly minimized as compared with conventional rotary cone type drill bits.